Apparatus for ultrasound imaging are known and widely used, mainly for 3D breast scanning
3D ultrasound imaging of a breast placed in a liquid bath, preferably in water, has certain advantageous characteristics, for example, distortions in structure detection may be prevented, and the process is inherently safer.
Nevertheless, the pulses emitted by the emitting unit or the emitting and receiving unit are required to pass through at least two interfaces made of materials having different acoustic impedances.
Particularly, when water is the acoustic impedance adaptation medium, a water-skin interface is added, which has a high acoustic impedance mismatch.
A number of undesired effects occur at this interface, including high reflection, which involves lower energy transmission, and high refraction, which involves divergence of the acoustic beams from the ideal straight path.
These effects, which can be found in prior art apparatus, lead to a decrease of sensitivity, and affect spatial resolution of images.
In prior art apparatus, this problem is yet unsolved and the need still exists for technical solutions that might limit reflection and refraction.
These apparatus also showed a new scanning problem, which was not found in standard handheld ultrasound probes.
If the transmitted pulses have an angle of incidence of more than 60°, the signal intensity is attenuated.
The worst condition occurs when the angle of incidence is close to a critical angle of about 66°, which somewhat creates a threshold effect.
A prior art apparatus is disclosed, for instance, in U.S. Pat. No. 7,771,360, incorporated herein by reference, in which the emitting and receiving unit performs scans on planes that are substantially parallel to the coronal plane of the patient.
This leads to a general degradation of image quality in the areas in which the angles of incidence fall close to the above values, and particularly in the area of the nipple, the trunk wall, or in case of a small breast.